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We have been talking about glaciers. One way to think about glacier is, they are rivers of ice.

But sometimes glaciers can be much larger than rivers. They can be
oceans of ice, like the glaciers that cover Antarctica and Greenland.

What would an ocean of ice do to the land beneath it? What would the land look like when the glaciers melt away?

Let’s start by thinking about rocks in fast-moving streams.

Stones in mountain streams are often round and smooth. How does that happen?

As the water moves them, they bump into other stones. Their corners
and edges get knocked off, and they get round as they roll down the
stream. As sand rubs against them –and they rub against sand– the
smallest edges and corners get knocked off too, and they become polished
and smooth. That takes a lot of rolling, so it takes a lot of time.

Now look at these pictures of mountains. The mountains on the bottom
are all jagged and sharp. The mountains on the top are more round, more
smooth.

So which of these mountains are older?

The ones that are round and smooth are older! Their edges have been worn off.

What could wear a mountain down? And how long does it take?

Water wears the mountain down. Water erodes it away. All those rocks
moving down streams and rivers do not move back up the mountain.

Sometimes the water is ice! A glacier! Glaciers are like bulldozers
–actually, several lines of very slow bulldozers– and push huge amounts
of rock downhill.

Back to our first two questions: What would an ocean of ice do to the
land beneath it? It would wear off the edges and corners of the land,
making it rounder.

What would the land look like when the glaciers melt away? It would look like the mountains in the top picture!

Those worn-down mountains are in the Hudson Highlands in New York.
There is good evidence that they were once covered completely by
glaciers, sometimes a mile deep! These mountains were worn down by ice,
lots and lots of ice, covering the whole landscape, pushing it down and
wearing it away.

The mountains on the bottom are from the Rocky Mountains, in
Colorado. They are still growing! They are very young compared to the
old, rounded mountains in New York.

How long does it take to wear down a mountain?

Well, the evidence shows that the rounded mountains in New York
stopped growing 250 million years ago. (That’s about 20 million years
before the first dinosaurs.) So a good answer is, it takes a couple of
hundred million years to wear a mountain down, to make a mountain smooth
and round instead of jagged and sharp.